1. Field of the Invention
The present invention relates to an engine and more particularly to gas turbine engines in which the gas turbine drives a shaft on which a propeller or unducted fan is provided for propulsion.
2. Description of Related Art
Operation of gas turbine engines is well known. In some situations it is preferable to couple the propulsion power of that gas turbine engine through a shaft to drive an unducted fan, open rotor or prop for propulsion. These engine configurations are referred to as propfans or contra-rotating fans and utilise a relatively low specific thrust engine. It is also known there are advantages with regard to positioning such propfans and contra-rotating fans towards the rear of an aircraft in a pusher or aft fan configuration.
A significant problem with these gas turbine engines is ingress of debris and birds as particulate matter which will impinge upon the blades of the compressor stage(s) and subsequent parts of the engine's core machinery with possibly significant impact damage or long term wear problems. It will be understood that one of the principal stages of a gas turbine engine's operation is to draw in or suck air which is compressed and then utilised within the thrust core of that engine in order to generate through combustion power either utilised directly in terms of thrust or, in accordance with aspects of the present invention, in order to drive a shaft upon which a propeller or unducted fan is located. Furthermore, it will be understood that a propeller located rearward will result in the inlet for the thrust core, that is to say the turbine machinery of the engine, being very exposed to debris, bird and other particulate matter.
FIG. 1 illustrates a prior prop fan or open rotor pusher configuration of an engine 1 in accordance with aspects of the present invention. It will be noted that a thrust core 2 comprises a number of compressor and turbine stages which drive a shaft 3 upon which propellers 4, 5 are located. The thrust core 2 has a core inlet 6 through which air is drawn in order to provide propulsion in accordance with known gas turbine operation. As air is drawn in through the inlet 6 it will be appreciated that there is exposure to debris, birds and other particulate matter. It will be understood that the engine 1 must be sufficiently robust to survive typical particulate matter ingress.
One approach to provide particulate debris separation is to a have a curved inlet duct such that the denser hard debris will be separable from the air flow and dumped through an appropriate overboard aperture. However, provision of such a curved inlet duct may not be convenient. Furthermore, some approaches have suggested provision of a plenum chamber within which the particulate matter is collected and then dumped at a convenient time, so avoiding potential problems with respect to loss of performance within the engine. In any event, as indicated, curved ducts do not lend themselves to full annular intakes and are more appropriate to “chin” or bifurcated inlets. It will also be understood that larger particulate matter such as birds would remain substantially intact requiring a fairly large catcher duct for overboard dumping.
Furthermore, proposals such as outlined in U.S. Pat. No. 3,979,903 include use of a booster rotor and a switchable particulate matter dump, but such arrangements are configured such that dump flow is not provided at take off when debris separation will typically be at its greatest necessity. Provision of a switchable dump effectively provides a control bleed flow to help part speed aerodynamic handling of the booster rotor which has a sufficiently high pressure ratio to mitigate for a shortfall in pressure ratio capacity in the high pressure core of the thrust part of the engine. In such circumstances continued bleeding in accordance with the proposals of U.S. Pat. No. 3,979,903 would be too penalising on overall engine operational efficiency.